This invention relates to a side sealing mechanism for a packaging machine of the type in which a flexible web of heat-sealable material is advanced and folded to form two face-to-face strips which are sealed together at longitudinally spaced positions to convert the web into series of interconnected pouches. The pouches subsequently are separated by cutting through the side seals and then are advanced through a filling station where product such as a food product is deposited into the pouches.
The invention has more particular reference to a sealing mechanism for a continuous motion packaging machine. In such a machine, the web, the folded strips and the severed pouches are advanced with continuous motion as the pouch forming and filling operations are performed. A typical continuous motion machine is disclosed in Nutting et al U.S. Pat. No. 3,230,687 and such a machine is capable of operating at significantly higher speeds than an intermittent motion machine in which the various operations are carried out when the web and the pouches dwell between successive steps.
In most instances, the web is pre-printed with a repeating pattern of labeling information, decorative artwork, advertising material and the like which ultimately appears on the outer sides of the finished pouches. In order for each pouch to have the proper appearance, it is necessary that the side seals register correctly with the printing on the web. In other words, each side seal should be located exactly at the proper area of repeat of the printed pattern on the web so that all pouches will be of proper and uniform appearance.
To keep the packaging machine operating in registration with the printing on the web, it is conventional for the web to be preprinted with photoelectric targets which are spaced longitudinally from one another in accordance with the width of the pouches. Photoelectric detectors sense the targets and produce signals which are used to control the operation of various mechanisms of the machine. Packaging machines in which photoelectric detectors sense preprinted targets to maintain web registration are disclosed in Lense U.S. Pat. No. 3,500,726, in Johnson et al U.S. Pat. No. 3,545,166 and in Johnson et al U.S. Pat. No. 3,553,934.
Difficulty is encountered in obtaining proper registration of the side seals because, in many instances, the printed pattern and the photoelectric targets may not repeat at exactly the same interval throughout the entire length of the web. Variations in the repeat interval may be caused by the web being stretched or tensioned by different amounts during its advance, by temperature fluctuations, by imperfections in the printing process itself and by other factors. While the variation of the repeat interval within a given group of pouches may be very minute, the cumulative variations that may occur in a web which is several hundred feet in length can result in the side seals being formed out of registration with the printed material and thereby ruin the appearance of the pouches.